A formerly necessary wagon attached to the back of a train. The brake van was used to stop a train in case of an emergency (such as if a coupling had snapped, and the latter half of the train was rolling down a steep hill). On longer journeys, the brake van served as a temporary residency for the train crew.
Nowadays, most trucks (US:freightcars) are already equipped with brakes so they'll stop automatically should something go wrong. Some trains even have locomotives at both ends, further reducing the necessity of a brake van.
Synonyms: caboose(US), guard's van
Driver: Cor! Look at that!!
*trucks rocket down a steep hill*
Fireman: I hope the guard (US: conductor) knows how to work the brake!!
Meanwhile, the guard sees a train coming, and he cranks the brake handle like mad. We see sequences of footage as the train and the brake van get closer and closer.
The guard, seeing his whole life flash before his eyes, shuts his eyes and waits for the impending crash-
Driver 2: Oi, mate, what're you doing? Gonna rub noses with our train for the day or what?
The guard notices he's still alive.
Guard: *wipes forehead* Phew! Thank God we had a brake van; this could have been quite nasty!